1. Field of the Invention
The present invention relates to an improved voltage-unbalance detector for electrical systems and more specifically, to an automatic detector usable with switchgear in polyphase electrical systems which detector responds to phase-to-ground over-voltages phase-to-ground under-voltages on one or more phases of the polyphase electrical system, as well as to improper phase orientations among the voltages on the phases of the polyphase electrical system.
2. Brief Description of the Prior Art
Numerous control systems and detectors for polyphase electrical systems and for equipment contained in such systems are well known. One type of equipment often used or found in polyphase electrical systems is metal-enclosed switchgear which usually contains a series combination, for each phase of the system, of an interrupter switch and a power fuse. An interrupter switch is an electrical switch which may be manually or automatically operated (opened or closed) as dictated by the requirements of the system or by requirements of loads connected thereto. An interrupter switch has the capability of opening a phase in which it is contained and, upon such opening, interrupting load currents and currents of lesser magnitude flowing in the phase. Such interruption normally includes the extinguishment or suppression of a high-voltage arc which forms when opening of the phase is attempted. The arc is extinguished or suppressed in a controlled environment so as to cause little if any distress to the switch itself, to the metal-enclosed switchgear in which this switch is contained, or to the electrical system to which the switchgear is connected.
The power fuses serially connected to each interrupter switch are essentially protective devices. Specifically, these power fuses respond to fault currents or other over-currents flowing in the phase in which they are connected and interrupt such fault currents or other over-currents upon the occurrence thereof. Interruption effected by the power fuses involves the extinguishment or suppression of the high-voltage arc which is formed incident thereto in a controlled environment so as to cause little if any distress to the electrical system or to other equipment connected thereto.
Under certain conditions, including single phase-to-ground faults, only one power fuse in one of the phases of the metal-enclosed switchgear may operate. The operation of one such fuse does not effect current flow in the other phases to which the metal-enclosed switchgear is connected. Accordingly, polyphase loads connected to the output of the metal-enclosed switchgear may either not perform optimally or may be ultimately damaged by a condition known as single-phasing, as is well known. To date, the art has lacked a convenient, inexpensive and reliable detector or control system for sensing such single-phasing and for operating the metal-enclosed switchgear in response thereto to prevent damage to the polyphase loads. Additionally, to date, the art has not known a convenient, inexpensive and reliable detector or control system for responding to over-voltages or under-voltages in some but less than all phases of polyphase electrical systems and for appropriately thereafter operating the metal-enclosed switchgear to prevent damage to the electrical system or to those connected thereto. Lastly, to date, the art has not known a detector or control system which is convenient to use and inexpensive to manufacture and reliable, and which can efficiently respond to improper phase orientations among the voltages on the phase of the polyphase electrical system.
The provision of a detector or control system as generally described above is a primary object of the present invention.